Lithium-air cells do not need to store positive electrode active materials inside of the cells because they utilize, as a positive electrode active material, oxygen in the air that can be provided from outside of the cells. Therefore, a large amount of lithium metals that are negative electrode active materials can be charged in the cells to acquire extremely large discharge capacities, whereby the cells are expected to be driven for a long period of time.
For example, in Patent Document 1, a lithium-air (oxygen) cell is evaluated by producing an electrolyte solution in which 1 mol/L of lithium bis(trifluoro methanesulfonyl)imide (LiTFSI) is used as a solute, and a mixed solvent of ethylene carbonate and diethyl carbonate is used as an organic solvent.
In Patent Document 2, a lithium-air cell is produced with an electrolyte solution in which 1 mol/L lithium hexafluorophosphate (LiPF6) is used as a solute, and a mixed solvent of ethylene carbonate and diethyl carbonate is used as an organic solvent, and is evaluated.
In both documents, the produced lithium-air cells have extremely large discharge capacities compared to those of common lithium ion cells.
However, the organic solvent, which is one of the components of the electrolyte solution in the lithium ion cells of these documents, is not only volatile, but also has some miscibility with water, and thus these cells do not seem to be stable when they work for a long time. Specifically, while these cells work for a long time, their electrolyte solutions are predicted to be volatilized from the positive electrode sides so that the cell resistance increases. In addition, oxygen and water may enter the cells to corrode lithium metals that are negative electrodes and to generate hydrogen. These events may impair a long time discharge, which is the characteristic of lithium-air cells.
To solve such disadvantages, Patent Document 3 describes a lithium-air cell using a lithium ionic conductive solid electrolyte instead of an electrolyte solution. However, the lithium-air cell works only at a relatively high temperature, such as 95° C.
In contrast, Patent Document 4 describes a lithium-air cell using two types of electrolyte solutions in order to prevent water intrusion into the negative electrode. One of the electrolyte solutions is an electrolyte solution of an organic solvent placed on the negative electrode side, and the other is an electrolyte solution of an aqueous solution placed on the air electrode side. In the lithium-air cell, the electrolyte solution of an organic solvent placed on the negative electrode side and the electrolyte solution of an aqueous solution placed on the air electrode side are separated with a solid electrolyte (separator) that conducts only lithium ions. This inhibits water permeation from the air electrode side to the negative electrode side, offering a superior level of safety.
However, in this system, the electrolyte solution (of an organic solvent) on the negative electrode side and the electrolyte solution (of an aqueous solution) on the air electrode side are required to be prepared independently, and the electrolyte solutions are required to be separated from each other with the lithium ion solid electrolyte. Thus, manufacturing of the cell may become complicated. In addition, the capacity may be limited because the aqueous solution serves as an electrode active material for the air electrode.